Polymers may be prepared by solution polymerization, wherein monomer is polymerized in a solvent or diluent. Polymers may also be prepared by bulk polymerization (also called mass polymerization), wherein monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent.
Stereoregular polymers such as cis-1,4-polydienes can be produced by polymerizing monomers such as conjugated dienes in the presence of a coordination catalyst system. An example of a coordination catalyst system is a lanthanide-based catalyst system. Lanthanide-based catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents.
In certain situations, the polymer produced by a coordination catalyst system is reactive. As a result, it may be advantageous to further react the polymer with a functionalizing agent to yield a functionalized polymer with tailored properties. Once the functionalized polymer has been prepared, the polymerization mixture in which the polymer is contained may be quenched with a quenching agent in order to render the polymerization mixture non-reactive. For example, quenching the mixture can cease further polymerization of monomer as well as deactivate any residual reactive polymer, catalyst, and/or catalyst components. This quenching may take place by protonating the residual reactive polymer, catalyst, and/or catalyst components.
The ability to react a functionalizing agent with a polymer produced by a coordination catalyst system is often not trivial. The reactivity of the polymer can be impacted by a number of factors and is often unpredictable. Also, the rate of reaction between a functionalizing agent and a polymer produced by a coordination catalyst system can be very slow or may be impacted by competing reactions occurring within the polymerization mixture such as the polymerization of monomer.
The slow reaction rate and/or unpredictability of the reaction between a functionalizing agent and a reactive polymer presents several disadvantages. First, monomer conversion (and correspondingly molecular weight) cannot be easily controlled. This can be especially deleterious in bulk polymerization systems because temperature control can be very difficult and local hot spots may occur within the polymerization mixture which can result in polymer degradation, gelation, and/or discoloration. In the extreme case, uncontrolled acceleration of the polymerization rate can lead to disastrous “runaway” reactions. Similarly, uncontrolled monomer conversion can lead to polymer products of very high molecular weight that form gels. The undesirable build-up of insoluble gelled polymer can foul equipment, which reduces productivity and increases the cost of operation.
In order to avoid these problems in bulk polymerization systems, great care must be taken to properly time the introduction of the functionalizing agent to the polymerization mixture. And, the timing of the addition of a quenching agent to protonate and thereby deactivate the mixture must likewise be carefully timed. In some situations, the quenching agent must be introduced to the polymerization mixture before adequate time is allowed for the functionalizing agent to react with the polymer.
Because there are many advantages associated with functionalized polymers prepared with coordination catalyst systems, there is a need to improve upon the polymerization processes employed to make these functionalized polymers.